France's nuclear safety agency, The Institute for Radioprotection and Nuclear Safety. had worked out that the radiation most likely originated in the Urals but was spread at high levels across large areas of European Russia.

It said the release would have posed no health or environmental risks to people in European countries outside Russia.

But it added that food would have been contaminated at above maximum permitted levels of radiation for tens of kilometres from the point of origin.

The French report said: "The detection of Ruthenium-106 alone excludes the possibility of a release from a nuclear reactor which would result in the presence of other radionuclides. The origin of Ruthenium-106 is therefore to be found either in nuclear fuel cycle facilities or radioactive source production."

Mayak has been responsible for at least two of Russia's worst nuclear accidents.

In 2004, waste from the plant was dumped in the local Techa River.

In 2016, journalists from the Associated Press visited a village downriver from Mayak where doctors have recorded chromosomal abnormalities, birth defects and cancers at rates much higher than the Russian average.

The Russian Academy of Sciences' Nuclear Safety Institute, which oversees safety standards, has insisted that Mayak's nuclear waste processing system poses no danger to people living nearby

Greenpeace said it would petition the Russian Prosecutor General's office to investigate "a possible concealment of a radiation accident" and check whether public health was sufficiently protected.

Ruthenium-106, which is sometimes created as a product of nuclear fission, is a stable radioactive isotope of Ruthenium, a naturally occurring metal related to platinum.

A spokesman for Rosatom told Sky News: "Rosatom categorically confirms there have been no unreported accidents or reportable events on any of its nuclear sites. It also confirms that the recent Ru-106 emission which is being reported is not linked to any Rosatom site.

"Reports of high Ru-106 levels show that the highest concentrations are in areas outside of Russian territory.

"Recent comment may stem from a misreading of yesterday's statement from Russian meteorology service Rosgidromet, confirming relatively high levels of Ru-106 in the air some parts of Russia.

"The highest reported readings in the town of Argayash (Chelyabinsk Oblast) are still much lower than equivalent readings taken in Bucharest: 76 mBq/m3 compared to the one in Bucharest (Romania) of 145 mBq/m3, according to the IAEA.

"Rosatom is committed to the highest standards of transparency and is working closely with the IAEA and other international experts to identify the potential source of the emission."